1. field of the Invention
The present invention relates generally to an optical receiver including an optical coupler for mixing two signals and an electronic amplifier.
2. Description of the Related Art
An optical receiver is disclosed in IEE Colloquium on Advances in Coherent Optic Devices and Technologies, Savoy Place, London, Great Britain, March 1985 and in ECOC 1986, pages 395-398. Each of these disclosures shows an optical ASK multiport homodyne receiver wherein a transmission signal and an oscillator signal are brought together in a 3.times.3 optical coupler. Different, relative phase shifts of the two incident signals are available at the outputs of the optical coupler. The relative phase difference between the three mixed signal components which can be taken from the 3.times.3 coupler must respectively amount to 120 degrees, and in addition, the disclosed coupler is not easy to manufacture. The three mixed signal components are conducted to three separate amplifiers in an electronic amplifier portion.
Homodyne receivers as shown in Journal of Lightwave Technology, LT-3, 1985, pages 1238-1247, and multiport receivers as disclosed in Electronics Letters 21, 1985, pages 867-868 can be advantageously constructed, for example, with what are referred to as 90 degree hybrids. Optical hybrids for free beam propagation are known from AEU 37, 1983, pages 203-206 wherein a 90 degree shift of two orthogonal field strength components of a circularly polarized light source is exploited. In superheterodyne receivers for lightwave guide heterodyne systems, however, it is desirable to execute all optical components in wave guide technology.
A 90 degree hybrid is realized with four fiber directional couplers connected to one another and with a phase shifter as disclosed in the D. W. Stowe paper TUB 5, OFC-83, New Orleans.